The main objective of a yawing device in a wind power plant with a horizontal wind turbine axis is to position the turbine into the direction of the wind. The prior art yawing device comprises an electrical or hydraulic motor and a high-ratio gear which acts on the toothed path of the yaw bearing and thus turns the machinery into the desired position. Due to the influence of e.g. wind shear the machinery is subjected to pulsating forces, both when yawing and when the machinery is stationary. These forces often have a dominating tendency in one direction, which means that they tend to turn the wind turbine out of the direction of the wind. During yawing, coriolis forces are also added.
These phenomena appear regardless of the number of turbine blades, but are less dominant on turbines with three or more blades. In many cases the machinery has to include strong yaw brakes, which for some designs also have to be partly activated when yawing, in order to avoid the development of damaging oscillations. Such oscillations initially develop in yaw, but coupling with the rest of the machinery may result in oscillations also in the blades and in the tower. Since wind power plants have to be built very slender due to economical reasons, these tendencies are strong. If the machinery is locked to the tower by means of e.g. yaw brakes, the tower will be subjected to large yawing moments, while the tendency to oscillate remains.
Designs with or without yaw brakes may be supplemented with hydraulic dampers, composed of a hydraulic cylinder or a hydraulic pump/motor, the flow of which has to pass through a narrow orifice. The damping components may partly be identical with the hydraulic system that executes the yawing of the wind turbine.
The European patent application No. 0 110 807 A1 discloses a wind power plant where the impact of the damaging oscillations mentioned above are minimised. This is accomplished by installing a drive motor between the turbine of the wind power plant and the yaw bearing. This drive motor is combined with a narrow orifice and thus acts as a damper. However, it is well known in prior art that to achieve the damping effect with this drive motor, other components, such as valves, tanks, filters etc., have to be added.
Even if the mentioned drive motor and its adjoining elements solves the problem with the damping of the damaging oscillations, this solution has a number of drawbacks. This drive motor and its adjoining elements are expensive. It is shows complexity, i.e. installation and maintenance will be expensive and time consuming. Finally the availability will be affected, since the vulnerability of the device will increase with the number of elements added.